1. Field of the Invention
The present invention relates to an illuminating optical system which is adapted to be capable of illuminating samples or the like with a thin slit-like beam.
2. Description of the Prior Art
The conventional illuminating optical systems of this type are mainly classified into three types which are adapted:
(1) to prepare a slit-like beam by passing a collimated light through a slit and allowing the slit-like beam to fall directly onto a sample without transmitting said slit-like beam through an objective lens, (2) to form a contracted image of a slit by irradiating a slit with a light emitted from a light source and allowing the image of the slit to fall onto a sample , and (3) to form a fringe pattern in the vicinity of a sample by allowing two laser beams to interfere with each other and allow a portion having the highest light intensity of the fringe pattern to fall onto the sample through a slit placed immediately before the sample.
However, the optical system of the type (1) requires, for obtaining a thin slit-like beam, a very thin slit which is difficult to manufacture and increases loss of light amount in practice. Further, the optical system of the type (2) poses a problem that the operability deteriorates remarkably since the distance between the objective lens and the sample (working distance) shortens if the objective lens is intended to have high contraction ratio. Furthermore, when the optical system of the type (2) uses two objective lenses at the same time, these objective lenses will interfere with each other. Accordingly, the optical system of the type (2) prevents use of objective lenses having so high contraction ratios, and requires, for obtaining a slit-like beam, a thin slit not so extreme as the thin slit for the optical system of the type (1) , and poses the problem similar to that of the optical system of the type (1). Moreover, when an optical member functioning as a cylindrical lens (for example, a thin water flow) exists between the objective lens and the sample as in a case of light measurement using a flow cytometer (hereinafter abbreviated as F.C.M.), the optical system of the type (2) poses a problem that the optical member functioning as a cylindrical lens has a longitudinal refractive index which is different from the lateral refractive index thereof, and the optical system cannot form an image of the slit having favorably corrected aberrations only with the objective lens which is a coaxial system. Moreover, the optical system of the type (3) poses a problem that, when it uses diffracted beams, the fringe pattern is blurred unless the pattern is formed on the sample and the slit must be placed almost in contact with the sample, thereby remarkably degrading the optical system. In addition, the optical system of the type (3) uses only the central portion the fringe pattern and causes remarkable loss in the amount of light.